1. FIELD OF THE INVENTION
The present invention relates generally to an electron beam apparatus and more particularly to a specimen holder of an improved structure used in the electron beam apparatus.
2. DESCRIPTION OF THE PRIOR ART
As a typical one of the electron beam apparatus, there may be mentioned a transmission electron microscope whose known structure is shown in FIG. 1, by way of example. The illustrated transmission microscope is composed of a microscope column 1 which includes an electron gun 3, a condenser lens 5, an objective lens 9 provided with excitation coils 8 and 10 and disposed below the condenser lens 5, an intermediate lens 11 and a projection lens 12, both being located below the objective lens 9 and a viewing chamber enclosure 2 mounted fixedly below the microscope column 1 and defining a hollow chamber 13 in which a fluorescent screen 14 is disposed. In the microscope column 1, there is formed a center bore 7 which extends longitudinally along a beam axis 6 of electrons emitted by the electron gun (which axis will hereinafter be referred to as the optical axis). The objective lens 9 includes an upper magnetic pole piece 16 excited by the excitation coil 8 and a lower pole piece 17 disposed below the upper pole piece 16 with a predetermined distance thereto and excited by the excitation coil 10. A specimen holder device 20 is withdrawably inserted in the space defined between the upper pole piece 16 and the lower pole piece 17.
In the transmission electron microscope of the structure outlined above, many endevors have been made to improve and enhance performances of the microscope. For example, there is realized an electron microscope which exhibits such a high resolving power that atomic structures of solid specimens can be observed and which is thus generally called a high performance electron microscope. In the high performance electron microscope, aberrations of the objective lens 9 are decreased to possible minimum. For example, in the electron microscope in which the electron beam is accelerated at an accelerating voltage of 100 kV, spherical aberration Cs as well as chromatic aberration Cc of the objective lens is of the order of 1 mm (millimeter).
In this connection, the resolving power of the objective lens which plays a determinantive role in determination of performance of an electron microscope can theoritically be expressed as follows: EQU .delta.=0.65 Cs.sup.1/4 .lambda..sup.3/4 ( 1)
where
.delta.: revolving power (mm),
Cs: spherical aberration (mm) of an objective lens, and
.lambda.: wavelength of accelerated electron beam (mm).
As will be appreciated from the above expression, the wavelength of the electron beam must be shortened by increasing the accelerating voltage or the spherical aberration Cs has to be much decreased, in order to produce an image of atomic structure of a solid specimen with high fidelity. In the present state of the electron microscopes, the maximum resolving power is attained at 1000 kV (kilovolts) at which the wavelength .lambda. of the electron beam is 8.7.times.10.sup.-3 .ANG., while the spherical aberration Cs is in the range of 2 to 3 mm.
In order to shorten the wavelength .lambda. of the electron beam, the electron beam accelerating voltage has to be considerably increased, which means that intolerably high manufacturing cost will be involved. Under the circumstances, the number of the high performance electron microscopes manufactured in a year which allows the accelerating voltage of 1000 kV to be used amounts only to one or two over the whole world. Accordingly, if the resolving power of the electron microscope is to be increased without being accompanied by the economical difficulty, the spherical aberration Cs has to be reduced, as will be apparent from the expression (1). The most conceivable measures for reducing the spherical aberration will be to decrease the distance between the specimen supported on the specimen holder device 20 and the magnetic pole pieces of the objective lens 9 (this distance is referred to as the working distance) by decreasing correspondingly the distance between the upper and the lower magnetic poles (16, 17). This approach however gives rise to a new problem which will be described below.
In the first place, it should be noted that a specimen holding or supporting portion of the specimen holder device 20 is at least 2 mm in thickness as viewed in the axial direction of the microscope. This thickness of this order is inevitable in consideration of the fact that a specimen mesh is fixedly disposed on the holding portion, a manipulating member for inclining or angularly positioning the specimen must be provided, and that the specimen holder should exhibit a sufficiently high anti-vibration capacity for attaining the high resolving power which allows a specimen image to be observed at the atomic level. Since the specimen holder device 20 is placed in and removed from the inter-pole gap of the objective lens 9 in the direction traversing the optical axis of the electron microscope (a so-called side entry system), the inter-pole gap of the objective lens 9 (i.e. the distance or space between the upper and the lower pole pieces of the objective lens) can not be decreased to a value smaller than 2 mm. For this reason, the value of the inter-pole distance is about 2 mm at the shortest in the hitherto known electron microscope. In other words, no attempts have been made to make the inter-pole gap of the objective lens smaller than 2 mm. Under the circumstance, the spherical aberration Cs of the objective lens 9 which is smaller than 1 mm is not realized, although it can be attained by decreasing the inter-pole gap below 2 mm.
As an approach to solve the problem mentioned above, there is known a so-called top entry system in which the specimen holder is placed in and withdrawn from the inter-pole gap of the objective lens 9 along the optical axis through the bore 7 of the upper pole piece 16 or the lower pole piece 17. With this arrangement, the inter-pole gap of the objective lens can certainly be decreased. However, the diameter of the bore 7 formed in the pole piece must then be greater than 5 mm in practical applications, which in turn results in that the spherical aberration becomes undesirably significant, to another disadvantage.